It is known to use diodes in parallel with on-off security switches, ac excitation of the system and separate reading of the effects of the positive and negative wave portions of ac excitation. These two portions give different effects, since the diodes are each low impedance devices only for one of the two exciting wave portions. Such a system is known from British Pat. No. 913,666 with contacts shunted by diodes. Some of the diodes check connections to circuit security switches with one polarity of the ac wave and the other diodes read for an alarm condition using the alternate polarity portions. This prior proposal provides some economies in resources such as cables.
It is known also to use plural voltage references such as zener diodes of different value to signal different alarm locations selectively, which can also save components in alarm systems generally. Eg see U.S. Pat. No. 3,676,877 (Kobayashi). This patent discloses shunt paths across a common line, of which each path has an on-off device responsive to a respective fire-sensor and in series with a zener device. The zeners have different values, so that a dc reading signal develops a voltage across the common line of a value which is indicative of which sensor has been activated. One cable input can thus serve many paths.
The use of end-of-cable or terminating zeners to indicate damage in a cable feeding a security network is known e.g. from U.K. Pat. No. 1,002,291 or U.S. Pat. No. 3,797,008 (Yuasa). U.K. No. 1,002,291 indeed gives different voltage indications by means of a series resistor and a voltage indicator at the input end, for open-circuit, short-circuit, normal and fire or abnormality alarm conditions, by means of shunt paths each having an on-off alarm contact and zener. Again, one voltmeter and cable can distinguish many alarm states.